Overcoming interfacial affinity issues in natural fiber reinforced polylactide biocomposites by surface adsorption of amphiphilic block copolymers

This work demonstrates that the interfacial properties in a natural fiber reinforced polylactide biocomposite can be tailored through surface adsorption of amphiphilic and biodegradable poly (ethylene glycol)-b-poly-(L-lactide) (PEG-PLLA) block copolymers. The deposition from solvent solution of PEG-PLLA copolymers onto the fibrous substrate induced distinct mechanisms of molecular organization at the cellulosic interface, which are correlated to the hydrophobic/hydrophilic ratios and the type of solvent used. The findings of the study evidenced that the performance of the corresponding biocomposites with polylactide were effectively enhanced by using these copolymers as interfacial coupling agents. During the fabrication stage, diffusion of the polylactide in the melt induced a change in the environment surrounding block copolymers which became hydrophobic. It is proposed that molecular reorganization of the block copolymers at the interface occurred, which favored the interactions with both the hydrophilic fibers and hydrophobic polylactide matrix. The strong interactions such as intra- and intermolecular hydrogen bonds formed across the fiber−matrix interface can be accounted for the enhancement in properties displayed by the biocomposites. Although the results reported here are confined, this concept is unique as it shows that by tuning the amphiphilicity and the type of building blocks, it is possible to control the surface properties of the substrate by self-assembly and disassembly of the amphiphiles for functional materials.

Synthesis and self-assembly of hybrid amphiphilic block copolymers

This thesis focused on the synthesis and self-assembly of novel block copolymers for the purpose of drug delivery. The block copolymers achieved comprise of a synthetic block and a peptide block and self-assemble into nano sized particles which can act as drug containers.

Nano-capsules of amphiphilic poly (ethylene glycol)-block-poly (bisphenol A carbonate) copolymers via thermodynamic entrapment

The synthesis of amphiphilic poly(ethylene glycol)-block-poly(bisphenol A carbonate) (PEG-b-PC) block copolymer is presented here using a simple bio-chemistry coupling reaction between poly(bisphenol A carbonate) (PC) with a monomethylether poly(ethylene glycol) (mPEG-OH) block, mediated by dicyclohexylcarbodiimide/4-dimethylaminopyridine. This method inherently allows great flexibility in the choice of starting materials as well as easy product purification only requiring phase separation and water washing. Collective data from Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and modulated dynamic scanning calorimetry (MDSC) confirmed the successful attachment of the poly(ethylene glycol) (mPEG-OH) and poly(bisphenol A carbonate) (PC) blocks. The preparation of nano-capsules was carried out by sudden addition of water to PEG-b-PC copolymers dispersed in THF, resulting in the controlled precipitation (i.e. thermodynamic entrapment) of the copolymer. Nano-capsules as small as 85 nm ± 30 nm were produced using this simple and fast methodology. We also demonstrate that encapsulating a water-insoluble bisphenol A diglycidyl ether (DGEBA) epoxy resin is possible highlighting the potential use of these capsules as a chemical delivery system.

Nanostructure and hydrogen bonding in interpolyelectrolyte complexes of poly(ε-caprolactone)-block-poly(2-vinyl pyridine) and poly(acrylic acid)

Nanostructured poly(ε-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP)/poly(acrylic acid) (PAA) interpolyelectrolyte complexes (IPECs) were prepared by casting from THF/ethanol solution. The morphological behaviour of this amphiphilic block copolymer/polyelectrolyte complexes with respect to the composition was investigated in a solvent mixture. The phase behaviour, specific interactions and morphology were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), dynamic light scattering (DLS) and atomic force microscopy (AFM). Micelle formation occurred due to the aggregation of hydrogen bonded P2VP block and polyelectrolyte (PAA) from non-interacted PCL blocks. It was observed that the hydrodynamic diameter (D_h) of the micelles in solution decreased with increasing PAA content up to 40 wt%. After 50 wt% PAA content, D_h again increased. The micelle formation in PCL-b-P2VP/PAA IPECs was due to the strong intermolecular hydrogen bonding between PAA homopolymer units and P2VP blocks of the block copolymer. The penetration of PAA homopolymers into the shell of the PCL-b-P2VP block copolymer micelles resulted in the folding of the P2VP chains, which in turn reduced the hydrodynamic size of the micelles. After the saturation of the shell with PAA homopolymers, the size of the micelles increased due to the absorption of added PAA onto the surface of the micelles.

Large compound vesicles from amphiphilic block copolymer/rigid-rod conjugated polymer complexes

Morphology evolution in complexes of amphiphilic block copolymers poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) and poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO) in the presence of polyaniline (PANI) in aqueous solution is reported. Transmission electron microscopy, atomic force microscopy, and dynamic light scattering techniques were used to study the morphologies at various PANI contents [aniline]/[acrylic acid] ([ANI]/[AA]) ranging from 0.1 to 0.7. The interpolyelectrolyte complex formed between PAA and PANI plays a key role in the morphology transformation. Spherical micelles formed from pure block copolymers were transformed into large compound vesicles upon increasing PANI concentration due to internal block copolymer segregation. In addition to varying PANI content, the kinetic pathway of nanoparticle formation was controlled through different water addition methods and was critical in the formation of multigeometry nanoparticles.

Flower like micellar assemblies in poly(styrene)-block-poly(4-vinylpyridine)/poly(acrylic acid) complexes

The formation of rare flower like micelles in poly(styrene)-block-poly(4-vinyl pyridine)/poly(acrylic acid) (PS-b-P4VP/PAA) diblock copolymer/homopolymer complexes is reported. The self-assembly as well as the morphological changes in the complexes were induced by the addition of a high molecular weight PAA/ethanol solution into the PS-b-P4VP solution in dimethyl formamide followed by dialyses. The composition-dependent micelles were varying in size and shape with increase in PAA concentration in solution. The complex aggregates in solution were characterized by dynamic light scattering (DLS) whereas morphologies in the solid complexes were observed using transmission electron microscopy (TEM). Flower like micelles are formed in complexes at 20 wt% PAA concentration followed by 'spikey' micellar assemblies at 40 wt% PAA. The size of the micelles was found to be increased upon the addition of PAA into the block copolymer solution. Infrared studies revealed the intermolecular hydrogen bonding interactions between the complementary binding sites on PAA and the P4VP block of the block copolymer. Finally, a model was proposed to explain the self-assembly and morphological transitions in these complexes based on the experimental results obtained.

A facile method to fabricate carbon nanostructures via the self-assembly of polyacrylonitrile/poly (methyl methacrylate-b-polyacrylonitrile) AB/B' type block copolymer/homopolymer blends

The self-assembly and high temperature behavior of AB/B′ type block copolymer/homopolymer blends containing polyacrylonitrile (PAN) polymers were studied for the first time. Here, microphase separated nanostructures were formed in the poly(methyl methacrylate-b-polyacrylonitrile) (PMMAN) block copolymer and their blends with homopolymer PAN at various blend ratios. Additionally, these nanostructures were transformed into porous carbon nanostructures by sacrificing PMMA blocks via pyrolysis. Spherical and worm like morphologies were observed in both TEM and AFM images at different compositions. The thermal and phase behavior examinations showed good compatibility between the blend components in all studied compositions. The PAN homopolymer (B′) with a comparatively higher molecular weight than the corresponding block (B) of the block copolymer is expected to exhibit ‘dry brush’ behavior in this AB/B′ type system. This study provides a basic understanding of the miscibility and phase separation in the PMMAN/PAN system, which is important in the nanostructure formation of bulk PAN based materials with the help of block copolymers to develop advanced functional materials.

Hydrogen bonding induced vesicular morphologies in diblock copolymer complexes

It is well-known that the self-assembly of block copolymers either in water or in organic solvents can form a wide range of morphologies in nanometer dimensions depending on its chemical nature. In the present study, the complexation and aggregate morphologies in a model AB/AC diblock copolymer system consisting of polystyrene-block-poly(acrylic acid) (PS-b-PAA) and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in water were studied using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and dynamic light scattering (DLS). By varying the relative amounts of the two block copolymers, a variety of bilayer aggregates were formed, including vesicles, multilamellar vesicles (MLVs), thick-walled vesicles (TWVs), interconnected compound vesicles (ICCVs), and irregular aggregates. The hydrophobic PS blocks were segregated as the cores while the hydrogen bonded PEO and PAA blocks formed the coronae of bilayer aggregates. We also investigate how the addition of PS-b-PEO into PS-b-PAA solutions influences the aggregate morphology of the resulting complexes. This work introduces a viable route to multicompartment vesicles in aqueous solutions. The formation of block copolymer vesicles in water is of particular interest because of their potential in various applications.

Self-assembly and morphology in double crystalline diblock copolymer/homopolymer blends with competitive hydrogen bonding

Block copolymers are of particular interest due to their ability to form a rich variety of nanostructures via self-assembly [1]. The self-assembly via competitive hydrogen bonding is a novel concept which is based on the competition between different blocks of the block copolymer to form more than one kind of intermolecular interaction with the complimentary polymer in the system. Recently, Guo and co-workers have proven that careful selection of the polymers specifically the block copolymer, and the experimental conditions can lead to self-assembled structures in blends and complexes exhibiting competitive hydrogen bonding [2-5].